The invention describes a silver-hafnium alloy composition, and more particularly, a silver-hafnium braze alloy composition, a bonded ceramic-metal and ceramic-ceramic article and a method of making said article.
Brazing is uniquely suited to the fabrication of ceramic-to-ceramic and ceramic-to-metal joints and seals due to the introduction of a liquid phase at the interface that facilitates reactions, diffusion and stable chemical bonds. Structural ceramics are among the most stable compounds known. A result of the chemical stability of ceramics is that they are difficult for liquid metals to wet. Because wetting is necessary for producing useable braze joints, an essential consideration in ceramic brazing is the need to promote wetting of the ceramic surfaces by the braze filler metals.
The conventional practice of brazing ceramics to ceramics or to metals involves a two step metallizing method in which a thin layer of metal is bonded to the ceramic component to improve the wettability of the ceramic surfaces by conventional low temperature filler metals. The metals are usually deposited by electroplating; however, in certain cases the coatings are produced by reducing oxides or sintered metal powder techniques.
The most widely known ceramic brazing method is the xe2x80x9cmoly-manganesexe2x80x9d (Moxe2x80x94Mn) method in which paint comprised of Mo and Mn powder is applied to the ceramic, generally Al2O3 or BeO, and fired in a controlled hydrogen atmosphere to create a strongly adherent viscous melt composed of metals and residual oxides not completely reduced by sintering. This surface is usually plated with a 2 to 4 micrometers thick layer of Ni or Cu, providing a surface that can be wetted easily by filler materials. Tsuno (U.S. Pat. No. 4,485,150, issued on Nov. 27, 1984) describes such a method for bonding a metal body and a ceramic body by adhering a metallizing layer to a ceramic body surface by heating under wet hydrogen atmosphere, nickel plating the surface and brazing by means of a silver-copper brazing alloy.
A more direct approach to ceramic/metal joining involves the use of active metal braze alloys. These alloys have the potential to eliminate numerous process steps involved, such as the need for Moxe2x80x94Mn metallization and subsequent Ni plating of alumina ceramics prior to brazing. Most commercial active metal braze alloys use Ti as an active element, with the most widely used composition being 63 wt % Agxe2x80x9435.25Cuxe2x80x941.75Ti. For many applications, Ti is an excellent active element and it reacts adequately with many non-metallic materials. However, there are certain metallic alloys, used extensively for electronic applications, where the Ti is scavenged by the dissolution of Fe, Ni, and Co into the braze joint, resulting in diminished amounts of Ti for reaction with the ceramic side of the joint. This can result in non-hermetic joints, or relatively low production yields.
Useful would be a metal braze alloy with an active element that allows use in a wider range of ceramic substrates.